Q-switched microlaser

ABSTRACT

A Q-switched microlaser is provided that is capable of supporting a zig-zag resonation pattern in response to pumping of the active gain medium so as to effectively lengthen the microresonator cavity without having to physically lengthen the microresonator cavity. As such, the microlaser can generate pulses having greater pulse widths and correspondingly greater pulse energies and average power levels than the pulses provided by conventional microlasers of a similar size. A corresponding fabrication method is also provided that permits a plurality of Q-switched microlasers to be fabricated in an efficient and repeatable manner.

RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/056,162 filed Jan. 24, 2002, which is adivisional of U.S. patent application Ser. No. 09/337,432 filed Jun. 21,1999 and now issued as U.S. Pat. No. 6,377,593, the contents of both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to microlasers andassociated fabrication methods and, more particularly, to Q-switchedmicrolasers and associated fabrication methods.

BACKGROUND OF THE INVENTION

[0003] Modern electro-optical applications are demanding relativelyinexpensive, miniaturized lasers capable of producing a series ofwell-defined output pulses. As such, a variety of microlasers have beendeveloped which include a microresonator and a pair of at leastpartially reflective mirrors disposed at opposite ends of themicroresonator to define a resonant cavity therebetween. Themicroresonator of one advantageous microlaser includes an active gainmedium and a saturable absorber that serves as a Q-switch. See, forexample, U.S. Pat. No. 5,394,413 to John J. Zayhowski, which issued onFeb. 28, 1995, the contents of which are incorporated in their entiretyherein. By appropriately pumping the active gain medium, such as with alaser diode, the microresonator will emit a series of pulses having apredetermined wavelength, pulse width and pulse energy.

[0004] As known to those skilled in the art, the wavelength of thesignals emitted by a microlaser is dependent upon the materials fromwhich the active gain medium and the saturable absorber are formed. Incontrast, the pulse width of the laser pulses emitted by a conventionalmicrolaser is proportional to the length of the resonator cavity. Assuch, longer resonator cavities will generally emit output pulses havinggreater pulse widths. Further, both the pulse energy and average powerprovided by a microlaser are proportional to the pulse width of thepulses output by the microlaser. All other factors being equal, thelonger the microresonator cavity, the longer the pulse width and thegreater the pulse energy and average power of the resulting laserpulses.

[0005] Conventional microlasers, such as those described by U.S. Pat.No. 5,394,413, are end pumped in a direction parallel to thelongitudinal axis defined by the resonator cavity. In this regard, thelongitudinal axis of the microresonator cavity extends lengthwisethrough the resonator cavity. Since the resonation cavity is generally arectangular solid, the longitudinal axis is oriented so as to beorthogonal to the pair of at least partially reflective mirrors thatdefine the opposed ends of the resonant cavity. As such, conventionalmicrolasers are configured such that the pump source provides pumpsignals in a direction perpendicular to the at least partiallyreflective mirrors that define the opposed ends of the resonant cavity.The effective length of the resonator cavity is therefore equal to thephysical length of the resonator cavity.

[0006] While the microlaser can be fabricated such that the resonatorcavity has different lengths, a number of factors contribute togenerally limit the permissible length of the resonator cavity. See, forexample, U.S. Pat. No. 5,394,413 that states that the resonator cavity,including both the saturable absorber and the gain medium, is preferablyless than two millimeters in length. In particular, a number ofelectro-optical applications require microlasers that are extremelysmall. As such, increases in the length of the resonator cavity arestrongly discouraged in these applications since any such increases inthe length of the microresonator cavity would correspondingly increasethe overall size of the microlaser.

[0007] In addition, the length of passively Q-switched microlasers iseffectively limited by the requirement that the inversion density mustexceed a predetermined threshold before lasing commences. As thephysical length of the resonator cavity increases, greater amounts ofpump energy are required in order to create the necessary inversiondensity for lasing. In addition to disadvantageously consuming morepower to pump the microlaser, the increased pumping requirements createa number of other problems, such as the creation of substantially moreheat within the microlaser which must be properly disposed of in orderto permit continued operation of the microlaser. In certain instances,the heat generated within the microlaser may even exceed the thermalcapacity of the heat sink or other heat removal device, therebypotentially causing a catastrophic failure of the microlaser.

[0008] Since the pulse width and correspondingly the pulse energy andaverage power of the pulses output by a microlaser cavity areproportional to the length of the resonator cavity, the foregoingexamples of practical limitations on the length of the resonator cavityalso disadvantageously limit the pulse width and the corresponding pulseenergy and average power of the pulses output by conventionalmicrolasers. However, some modem electro-optical applications arebeginning to require microlasers that emit pulses having greater pulsewidths, such as pulse widths of greater than 1 nanosecond and, in someinstances, up to 10 nanoseconds, as well as pulses that have greaterpulse energy, such as between about 10 μJ and about 100 μJ, and greateraverage power, such as between 0.1 watts and 1 watt. As a result of theforegoing limitations on the length of the resonator cavity and thecorresponding limitations on the pulse widths, pulse energy and averagepower of the pulses output by the conventional microlasers, conventionalmicrolasers do not appear capable of meeting these increased demands.

SUMMARY OF THE INVENTION

[0009] A microlaser is therefore provided according to one embodiment ofthe present invention that is capable of supporting a zig-zag resonationpattern in response to pumping of the active gain medium so as toeffectively lengthen the microresonator cavity without having tophysically lengthen the microresonator cavity. As such, the microlaserof these embodiments can generate pulses having greater pulse widths andcorrespondingly greater pulse energies and average power levels than thepulses provided by conventional microlasers of a similar size.

[0010] According to the present invention, the microlaser includes amicroresonator having an active gain medium and a Q-switch, such as apassive Q-switch proximate to and, in one embodiment, immediatelyadjacent to the active gain medium. In advantageous embodiments, theactive gain medium and the Q-switch are integral such that themicroresonator may be a monolithic structure. The microresonator extendslengthwise between opposed end faces. The microlaser also includes firstand second reflective surfaces disposed proximate respective ones of theopposed end faces to define a microresonator cavity therebetween. Whilethe first and second reflective surfaces can be coated upon respectiveones of the opposed end faces of the microresonators, the first andsecond reflective surfaces can also be formed by mirrors that are spacedfrom respective ones of the opposed end faces. The microlaser can alsoinclude a pump source for introducing pump signals into the active gainmedium via at least one of the end surfaces of the microresonator suchthat the zig-zag resonation pattern is established within themicroresonator cavity.

[0011] In one advantageous embodiment, the opposed end faces are eachdisposed at a nonorthogonal angle α, such as between about 30° and about45°, relative to a line perpendicular to a longitudinal axis defined bythe microresonator cavity and extending between the opposed end faces.In one embodiment, the opposed end faces are each disposed at the samenonorthogonal angle α relative to the longitudinal axis such that theopposed end faces are parallel. In another embodiment, the opposed endfaces are oriented in opposite directions by the same nonorthogonalangle α. As a result of the nonorthogonal relationship of the opposedend faces, the microlaser of either embodiment is capable of supportingthe zig-zag resonation pattern in response to pumping of the active gainmedium via at least one of the end surfaces of the microresonator.

[0012] By supporting the zig-zag resonation pattern, the effectivelength of the microresonator cavity is increased relative toconventional microlasers having substantially the same physical sizethat do not support a zig-zag resonation path. In this regard, theeffective length of the microresonator cavity of the present inventionis the length of the zig-zag resonation path established by themicrolaser which is significantly longer than the linear resonationpaths established by conventional microlasers that extend parallel tothe longitudinal axis of the resonator cavity. As such, the microlaserof the present invention can emit pulses having a longer pulse width andcorrespondingly greater pulse energies and average power levels than thepulses emitted by conventional microlasers of the same physical size.

[0013] In order to permit the pump signals to be received by the activegain medium without being reflected from the end face, the microlasercan include an antireflection coating on the end face through which thepump signals are delivered for permitting pump signals having apredetermined range of wavelengths to be received by the active gainmedium. The microresonator also generally includes a plurality of sidesurfaces extending between the opposed end faces. In order to furtherfacilitate resonation within the microresonator cavity, the plurality ofside surfaces can be roughened, such as by grinding, to thereby diffuselight.

[0014] In order to permit the microlaser to emit signals of apredetermined lasing wavelength via one of the opposed end faces, thefirst reflective surface is preferably highly reflective for lasersignals having the predetermined lasing wavelength. In contrast, thesecond reflective surface is preferably only partially reflective forlaser signals having the predetermined lasing wavelength. As such, themicrolaser can emit laser pulses having the predetermined lasingwavelength via the second reflective surface.

[0015] In one embodiment, the microlaser also includes a heat sink uponwhich at least the microresonator is mounted and a housing in which atleast the microresonator is disposed. In this embodiment, the housingincludes a window through which laser signals generated by themicroresonator are emitted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a side elevational view of a microlaser according to oneadvantageous embodiment of the present invention.

[0017]FIG. 2 is a side elevational view of a microlaser according toanother embodiment of the present invention.

[0018]FIG. 3 is a side elevational view of a microlaser according to oneembodiment of the present invention in which the microresonator and thepump source are disposed within a housing and in which a portion of thehousing has been removed to permit interior portions of the housing tobe depicted.

[0019]FIG. 4 is a perspective view of a composite structure comprised ofa passive Q-switch material and an active gain medium fabricatedaccording to a method of one embodiment of the present invention.

[0020]FIG. 5 is a plan view illustrating the composite structure of FIG.3 being divided into a plurality of bars according to the method of oneembodiment of the present invention.

[0021]FIG. 6 is a plan view illustrating a plurality of bars being cutat a nonorthogonal angle α relative to the opposed major surfacesaccording to the method of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0023] Referring now to FIG. 1, a microlaser 10 according to oneadvantageous embodiment of the present invention is illustrated. Themicrolaser includes a microresonator having an active gain medium 12 anda Q-switch 14, such as a passive Q-switch, proximate to the active gainmedium. Typically, the active gain medium and the passive Q-switch areimmediately adjacent to one another. However, the active gain medium andthe passive Q-switch may be proximate one another even though one ormore intervening layers may be disposed between the active gain mediumand the passive Q-switch. Additionally, the active gain medium and thepassive Q-switch are preferably integral and, in at least someembodiments, form a monolithic structure.

[0024] While the microresonator of one advantageous embodiment isfabricated by epitaxially growing the active gain medium 12 upon theQ-switch 14 as described below, the microresonator can be fabricated byepitaxially growing the Q-switch upon the active gain medium or in othermanners. For example, the active gain medium and the Q-switch can bejoined by a diffusion bond or by optical contact in which the activegain medium and the Q-switch are attracted with coherent forces, such asVan der Waals forces. In order to securely join the active gain mediumand the Q-switch by diffusion bonding or optical contact, the abuttingsurfaces of the active gain medium and the Q-switch must be extremelyclean and flat, such as to within {fraction (1/20)} of a referencewavelength, such as 633 nanometers in one exemplary embodiment

[0025] Both the Q-switch 14 and the active gain medium 12 are formed ofan appropriately doped host material. Typically, the host material isyttrium aluminum garnet (YAG), although materials such as yttriumvanadate (YVO₄) and yttrium lithium fluoride (YLF) can be employed. Inaddition, while a variety of dopants can be utilized, the active gainmedium is typically doped with neodymium (Nd) and the saturable absorberis typically doped with tetravalent chrome. In one advantageousembodiment, for example, the active gain medium is formed of YAG that isdoped with between about 2 and about 3 atomic percent of Nd. In thisembodiment, the Q-switch or saturable absorber is also formed of YAG andis doped with tetravalent chrome so as to have an optical density of0.03 to 0.1. As will be apparent, however, the active gain medium andthe saturable absorber can be doped with different atomic percentagesand different types of dopant without departing from the spirit andscope of the present invention. For example, the active gain medium maybe formed of YVO₄ that is doped with thulium, such as about 5% thulium,and the saturable absorber may be formed of YVO₄ that is doped withholmium, such as about 2% holmium.

[0026] Regardless of the material selection, the saturable absorberserves as a Q-switch 14 to prevent the onset of lasing until theinversion density within the microresonator is sufficiently high, i.e.,above a predetermined threshold. Once lasing begins, however, themicroresonator will produce a series of pulses of a predeterminedwavelength, i.e., the lasing wavelength, that have a predetermined pulsewidth, albeit a longer pulse width than the laser pulses generated byconventional microlasers.

[0027] The microresonator extends lengthwise between opposed end faces16. In the advantageous embodiment illustrated herein, the active gainmedium 12 is proximate one of the opposed end faces and the Q-switch 14is proximate the other end face. However, the active gain medium and theQ-switch can both extend lengthwise between the opposed end faces asdescribed in U.S. Pat. No. 6,219,361 entitled Side Pumped, Q-SwitchedMicrolaser, the contents of which are incorporated in their entiretyherein. The microlaser 10 also includes first and second reflectivesurfaces 18, 20 disposed proximate respective ones of the opposed endfaces to define a microresonator cavity therebetween. As shown in FIG.1, the first and second reflective surfaces can consist of amulti-layered dielectric coating that is deposited upon the opposed endfaces. Alternatively, the first and second reflective surfaces can beformed by first and second dichroic mirrors that are positionedproximate, but slightly spaced from respective ones of the opposed endfaces as shown in FIG. 2.

[0028] In either embodiment, the first reflective surface 18 proximatethe end face 16 of the microresonator defined by the active gain medium12 has a high reflectivity, such as a reflectivity of greater than99.5%, for signals having the predetermined lasing wavelength, such as1.064 nanometers for a microlaser having an active gain medium formed ofNd doped YAG. In addition, the second reflective surface 20 that isdisposed proximate the end face of the microresonator defined by thepassive Q-switch 14 is a partial reflector, typically having areflectivity of between 40% and 90% for signals having the predeterminedlasing wavelengths. See also U.S. Pat. No. 5,394,413 that furtherdescribes a pair of mirrors that define the resonator cavity of amicrolaser.

[0029] Once the active gain medium 12 is pumped such that the inversiondensity within the microresonator is above the predetermined threshold,the passive Q-switch 14 will permit a series of pulses to be emitted. Asa result of the partial reflectivity of the second reflective surface20, the series of pulses will then be emitted through the secondreflective surface.

[0030] The microlaser 10 also includes a pump source 22 for pumping theactive gain medium 12 with pump signals. The pump source may beconfigured to side pump the active gain medium 12 via one or more sidesurfaces as described by U.S. Pat. No. 6,219,361 and U.S. Pat. No.6,377,593, the context of both of which are incorporated in theirentirety herein. According to one advantageous embodiment, however, themicrolaser of the present invention is end pumped. In this regard, thepump source is preferably positioned such that the pump signals aredelivered via the end face 16 of the microresonator as described below.

[0031] Although the wavelength of the pump signals can be tailored tothe specific materials that comprise the active gain medium 12, anactive gain medium that is comprised of Nd doped YAG is typically pumpedwith pump signals having a wavelength of 808+/−3 nanometers. In order topermit the pump signals to be received by the active gain medium withoutbeing reflected from the end face 16, the microlaser generally includesan antireflection coating 26 deposited upon the end face 16 throughwhich the pump signals are introduced to permit signals having thewavelength of the pump signals to enter the microresonator cavity withlittle, if any, reflection. While the antireflection coating 26deposited upon the end face 16 can be formed in a variety of manners,the antireflection coating is typically formed by the deposition of aplurality of dielectric layers having respective indices of refractionthat are tailored to provide the proper reflectivity properties as knownto those skilled in the art.

[0032] While the microlaser 10 can include a variety of pump sources 22,the microlaser of one advantageous embodiment utilizes one or more laserdiodes. For example, the pump source may be a single stripe laser diodehaving an aperture of about 100 microns and capable of delivering 5-10watts of pump power. The output of the pump source, such as a laserdiode, may be focused onto end face 16 with a lens. Alternatively, thepump source, such as a laser diode, may be butt coupled to the end face16. Still further, the pump source, such as a laser diode, may be remotefrom the microresonator such that the output of the pump source isdelivered to the end face 16 by means of one or more optical fibers.

[0033] Since the microresonator is typically an elongate bar having agenerally rectangular cross-section (taken in a direction perpendicularto the longitudinal axis 28), the microresonator also typically includesfirst, second, third and fourth side surfaces 16. In FIGS. 1 and 2, forexample, the first side surface faces upwardly, the second side surfacefaces downwardly, the third side surface faces the viewer and the fourthside surface faces away from the viewer and is therefore unseen. Inorder to prevent much, if any, light from entering or departing from themicroresonator cavity via these side surfaces, the side surfaces aretypically finely ground or otherwise roughened so as to diffuse light.

[0034] As a result of the angled configuration of the end faces 16 ofthe microresonator as described below, the resonation patternestablished by the microresonator is not parallel to the longitudinalaxis 28 as is established by conventional microlasers that have amicroresonator that is a rectangular solid in shape. Instead, theresonation pattern established by the microresonator of the presentinvention is a zig-zag resonation pattern as shown in dashed lines inFIGS. 1 and 2.

[0035] In order to support the zig-zag resonation pattern established inthe microresonator cavity, the opposed end faces 16 of themicroresonator are each preferably disposed in a nonorthogonal mannerrelative to the longitudinal axis 28 defined by the microresonatorcavity. While the opposed end faces can be disposed at a variety ofnonorthogonal angles relative to the longitudinal axis, the opposed endfaces are typically disposed at an angle α that is between about 30° andabout 45° relative to a line perpendicular to the longitudinal axis and,more commonly, at an angle α of about 30.9°.

[0036] The microresonator cavity is preferably constructed such that thesignals undergo an integer number n of reflections or bounces from theside surfaces of the microresonator cavity prior to emission via thesecond reflective surface 20. In this regard, the length L_(t-t) of themicroresonator cavity measured tip-to-tip and the length L of a sidesurface are shown in FIGS. 1 and 2 and can be defined as follows:

L _(t-t)=(n×1)+(2×s)  (1)

L=(n×1)−(2×s)  (2)

[0037] wherein s is the length in a direction parallel to thelongitudinal axis 28 of one-half of an end face 16.

[0038] As will be noted, in instances in which the opposed end faces 16are parallel to one another, the length L of a side surface is identicalfor both the first and second side surfaces and the length L_(t-t) ofthe microresonator cavity measured tip-to-tip is longer than either ofthe first and second side surfaces as shown in FIG. 1. However, inembodiments in which the microresonator cavity has end faces that areoriented at the same angle but in opposite directions as shown in FIG.2, the length L of a side surface is the length of the shorter sidesurface, while the length L_(t-t) measured tip-to-tip of themicroresonator cavity is the length of the longer side surface.

[0039] Equations 1 and 2 can be rewritten in terms of other variablessuch as the thickness t of the microresonator cavity as measured betweenthe first and second opposed side surfaces, the angle of incidence (AOI)of the pump signals relative to a line perpendicular to the end face 16and the angle a of the end face as defined between the end face and thelongitudinal axis 28 extending through the microresonator cavity, i.e.,a=90°−α. In order to rewrite equations 1 and 2 in terms of these othervariables, the length l of one bounce the signals within themicroresonator cavity and the length s as measured along thelongitudinal axis of one half of the end face 16 are defined as follows:$\begin{matrix}{l = {t \times {{Tan}\left( {a + {{Sin}^{- 1}\left( \frac{{Sin}\left( {A\quad O\quad I} \right)}{n_{r}} \right)}} \right)}}} & (3) \\{{2s} = \frac{t}{{Tan}(a)}} & (4)\end{matrix}$

[0040] wherein n_(r) is the index of refraction of the microresonatorcavity and, more particularly, the host material, such as 1.818 forembodiments in which YAG is the host material.

[0041] By substituting equations 3 and 4 into equations 1 and 2, thelength L_(t-t) tip-to-tip and the length L of a side surface may beredefined as follows: $\begin{matrix}{L_{t - t} = {{n \times t \times {{Tan}\left( {a + {{Sin}^{- 1}\left( \frac{{Sin}\left( {A\quad O\quad I} \right)}{n_{r}} \right)}} \right)}} + \frac{t}{{Tan}(a)}}} & (5) \\{L = {{n \times t \times {{Tan}\left( {a + {{Sin}^{- 1}\left( \frac{{Sin}\left( {A\quad O\quad I} \right)}{n_{r}} \right)}} \right)}} - \frac{t}{{Tan}(a)}}} & (6)\end{matrix}$

[0042] In embodiments in which the end faces 16 of the microresonatorcavity are parallel to one another as shown in FIG. 1, the signal willundergo an even number of reflections or bounces, i.e., n is an evennumber. Alternatively, in embodiments in which the end faces disposed atthe same angle but in opposite directions as shown in FIG. 2, thesignals will undergo an odd number of reflections or bounces within themicroresonator cavity, i.e., n is an odd number. In either embodiment,the resulting microresonator cavity supports the zig-zag resonationpattern as shown.

[0043] By supporting a zig-zag resonation pattern within themicroresonator cavity, the effective length of the resonation pattern issignificant longer than the physical length of the microresonator cavityas measured along the longitudinal axis 28. In this regard, theeffective length of the resonation pattern is defined by the path of thesignals as the signals alternately bounce from the opposed side surfacesof the microresonator. For a microlaser 10 that is designed such thatthe signals reflect or bounce four times from the opposed side surfacesof the microresonator, i.e., n=4, the effective length of the zig-zagresonation pattern is about three to four times longer than the physicallength of the microresonator cavity as measured along the longitudinalaxis. Since the length of the resonation pattern and the physical lengthof the resonator cavity are identical for conventional end-pumpedmicrolasers, the microlaser of the present invention advantageouslyprovides a much longer resonation pattern without requiring thatphysical dimensions of the microresonator be increased.

[0044] As a result of the lengthened resonation pattern, the pulse widthor pulse duration of the pulses output by the microlaser 10 is increasedrelative to the pulse width of the pulses output by conventionalmicrolasers of the same size. For example, the pulses output by themicrolaser of the present invention are anticipated to have a pulsewidth of between 1 and 10 nanoseconds and, more typically, between about2 and 5 nanoseconds, as compared to the pulses output by conventionalend-pumped microlasers of the same size that do not support a zig-zagresonation path and which have subnanosecond pulse widths. In addition,the energy delivered by the pulses output by the microlaser of thepresent invention should be significantly greater than the energydelivered by the pulses output by conventional end-pumped microlasers ofthe same size. In this regard, pulses having an energy up to about 100μJ are anticipated to be emitted by the microlaser of the presentinvention in comparison to pulse energies of less than about 35 μJ thatare provided by the pulses output by conventional end-pumped microlasersof the same size that do not support a zig-zag resonation path.Correspondingly, the pulses emitted by the microlaser of the presentinvention are anticipated to have much greater average powers, such as0.1 watts to 1 watt, than the average power of conventional end-pumpedmicrolasers that is typically less than 0.1 watts.

[0045] While the microlaser 10 of the present invention can be packagedin a variety of manners, a packaged microlaser according to oneembodiment is illustrated in FIG. 3. As shown, the microlaser furtherincludes a heat sink 35 upon which the pump source 22 and themicroresonator are mounted. Although a variety of active and passiveheat sinks can be utilized, the heat sink of one advantageous embodimentis an oxygen free high conductivity copper heat sink. Regardless of thetype of heat sink, the pump source and the microresonator are preferablybonded to the heat sink by means of a thermally matched epoxy, such asan aluminum oxide filled or a silver filled epoxy.

[0046] The microlaser 10 of this embodiment also includes a housing 36in which the microresonator and the pump source 22 are disposed. Whilethe housing can be comprised of a variety of materials, the housing maybe comprised of a thermally conductive material and, in someembodiments, is comprised of the same material as the heat sink 35, suchas oxygen free high conductivity copper, in order to facilitatetransmission of the thermal energy to the heat sink for disposal.

[0047] As shown, the housing 36 includes a window 38 aligned with andtypically proximate to the second reflective surface 20 through whichpulses are output by the microresonator. The window is designed to betransmissive to signals having the predetermined lasing wavelength ofthe microresonator. As such, the pulses output by the microresonatorwill pass through the window with little, if any, attenuation. While thewindow can be constructed in a variety of manners, the window of oneadvantageous embodiment is comprised of sapphire and is coated with anantireflection coating that prevents little, if any, of the signalshaving the predetermined lasing wavelength from being reflected. Asdescribed above, the antireflection coating is typically formed of aplurality of dielectric layers tailored to have dielectric propertiesthat limit, if not prevent, reflection of signals having thepredetermined lasing wavelength. Although not necessary for the practiceof the present invention, the microlaser 10 can include a partiallyreflective mirror (not shown) for diverting a small fraction of eachoutput pulse to a power monitor, such as a photodetector, that monitorsthe output pulses so as to provide an indication if the microlaser failsto function properly.

[0048] Although the pump source 22 is depicted to be within the housing36 in the embodiment of FIG. 3, the pump source may be external to thehousing in other embodiments. As such, the housing may include anotherwindow through which the pump signals are introduced from an externalpump source. Alternatively, the pump source may be remote with the pumpsignals delivered via optical fibers that extend through the housing forilluminating an end face 16 of the microresonator cavity.

[0049] As will be apparent to those skilled in the art, the microlaser10 of the present invention is extremely advantageous in its ability todeliver pulses having longer pulse widths and greater pulse energiesthan the pulses delivered by conventional end-pumped microlasers ofsubstantially the same size that do not support a zig-zag resonationpath. As such, the microlaser of the present invention is advantageousfor a variety of applications, including marking, micromachining, LIDARand other ranging applications.

[0050] As described above, the microresonator can be fabricated in avariety of manners including epitaxially growing either the active gainmedium 12 or the passive Q-switch material 14 upon the other, diffusionbonding the active gain medium and the passive Q-switch or joining theactive gain medium and the passive Q-switch by optical contact. In oneparticularly advantageous embodiment, the active gain medium is grown,such as by liquid phase epitaxy, upon the passive Q-switch material. Assuch, the atomic percentage of the dopant in the active gain materialcan be significantly greater than the atomic percentage of dopant incomparable active gain material grown according to Czochralskitechniques. For example, the active gain medium of Nd doped YAG that isepitaxially grown upon a layer of tetravalent chrome doped YAG thatserves as the passive Q-switch material can have an atomic percentage ofNd that is between about 2 atomic percent and 3 atomic percent, incomparison to Nd doped YAG having an atomic percentage of Nd of 0.8% to1.4% if grown by a conventional Czochralski technique.

[0051] According to this advantageous fabrication technique, a layer ofpassive Q-switch material 14 such as tetravalent chrome doped YAG isinitially provided. Although the layer of passive Q-switch material canbe provided in a variety of forms, the layer of passive Q-switchmaterial is typically provided as a relatively thin wafer, which, in oneembodiment, has a thickness of about 500 microns. The active gain medium14 is then grown, preferably by liquid phase epitaxy, upon the layer ofthe Q-switch material to form the composite structure 42 shown in FIG. 4having opposed major surfaces 44. While the active gain medium can begrown so as to have a variety of thicknesses, the thickness of theactive gain medium is typically between about 2 and 4 millimeters, andin one embodiment, is 2.2 millimeters.

[0052] In the illustrated embodiment, the composite structure 42 is thencut into a plurality of lengthwise extending bars 46. See FIG. 5. Whilethe composite structure can be cut in a variety of manners withoutdeparting from the spirit and scope of the present invention, thecomposite structure is typically mounted to a glass plate with a layerof wax. In addition, the exposed major surface 44 of the compositestructure that is opposite the glass plate is also typically coated withwax to prevent shattering of the composite structure during the cuttingoperation. After placing the glass plate upon a vacuum chuck, thecomposite structure is cut into a plurality of bars with adiamond-tipped saw. While the bars can have a variety of thicknesses,the bars of one embodiment have a thickness designated T in FIG. 5 ofabout 1.2 millimeters. After removing the wax, each bar is laid on itsside and mounted to another glass plate 50 with an optical adhesive,such as Norland optical adhesive grade 65, as shown in FIG. 6. Afterplacing the glass plate upon a vacuum chuck, the bars are cut at anonorthogonal angle α relative to a line perpendicular to the opposedmajor surfaces to thereby form a plurality of passively Q-switchedmicrolasers 10. While the bars can be cut at a variety of angles asdescribed above, the nonorthogonal angle α is typically between about30° and 45° and, more typically is about 30.9°.

[0053] After removing the optical adhesive, the first, second, third andfourth side surfaces of the respective microlaser 10 can be roughened,such as by finely grinding the side surfaces, to thereby diffuse light.In the embodiment in which the first and second reflective surfaces 18,20 that define the opposed ends of the microresonator cavity aredeposited upon the opposed end faces 16 of the microresonator, themethod of this advantageous embodiment also contemplates depositing thefirst reflective surface that is highly reflective for signals havingthe predetermined lasing wavelength upon one end face 16 of themicroresonator and depositing the second reflective surface that is onlypartially reflective for signals having the predetermined lasingwavelength upon the other end face such that the resulting microlasersare capable of emitting signals of the predetermined lasing wavelengthvia the second reflective surface. As described above, the first highlyreflective surface is typically deposited upon the end face proximatethe active gain medium 12 and the second partially reflective surface istypically deposited upon the other end face proximate the passiveQ-switch 14. As described above, the first and second reflectivesurfaces are typically formed by depositing a series of dielectriclayers upon the opposed end faces of the microresonator that haverespective indices of refraction tailored to provide the appropriatereflectivity properties in a manner known to those skilled in the art.However, the first and second reflective surfaces can be formedaccording to other techniques without departing from the spirit andscope of the present invention. While the opposed end faces of themicrolaser can be coated prior to cutting the composite structure 42into a number of bars 46, the opposed end faces of the microlaser aretypically coated after the individual microlasers have been formed.

[0054] By constructing the microlaser 10 according to the foregoingmethod, the active gain medium 12 can be more heavily doped than theactive gain mediums of some conventional microlasers that are grownaccording to a Czochralski technique. As such, the output pulsesprovided by the resulting microlaser can have pulse energies and powerlevels that are even further increased relative to the output pulsesprovided by conventional microlasers. As described in detail above, themicrolasers of the present invention that are fabricated according tothe foregoing method or that are fabricated in other manners, such as bydiffusion bonding, are also particularly advantageous since themicrolasers provide output pulses having a greater pulse width or pulseduration and greater pulse energies and average power levels as a resultof the zig-zag resonation pattern supported by the microresonator incomparison to conventional end-pumped microlasers of substantially thesame physical size that do not support a zig-zag resonation pattern. Assuch, the resulting microlaser of the present invention is particularlyadvantageous for a wide variety of applications that are demand outputpulses having increased energy levels, average power levels and pulsedurations.

[0055] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A microlaser comprising: a microresonatorcomprising an active gain medium and a passive Q-switch proximate to andintegral with said active gain medium, said microresonator extendinglengthwise between opposed end faces; first and second reflectivesurfaces disposed proximate respective ones of the opposed end faces todefine a microresonator cavity therebetween; and a pump source forintroducing pump signals into the active gain medium via at least one ofthe end faces of said microresonator such that a zig-zag resonationpattern is established within the microresonator cavity.
 2. A microlaseraccording to claim 1 wherein said active gain medium is immediatelyadjacent said passive Q-switch.
 3. A microlaser according to claim 1wherein the passive Q-switch is adjacent one of the opposed end faces ofthe microresonator.
 4. A microlaser according to claim 1 wherein saidpump source introduces pump signals having a wavelength within apredetermined range of wavelengths, and wherein the microlaser furthercomprises an antireflection coating upon the end face through which thepump signals are delivered for permitting the pump signals to bereceived by the active gain medium without being reflected from thefirst side surface.
 5. A microlaser according to claim 1 wherein saidmicroresonator further comprises a plurality of side surfaces extendingbetween the opposed end faces, and wherein the plurality of sidesurfaces are roughened to thereby diffuse light.
 6. A microlaseraccording to claim 1 wherein the microresonator is adapted to generatelaser signals of a predetermined wavelength, and wherein said firstreflective surface is highly reflective for laser signals having thepredetermined wavelength while said second reflective surface ispartially reflective for laser signals having the predeterminedwavelength, thereby permitting laser signals to be emitted by themicrolaser via said second reflective surface.
 7. A microlaser accordingto claim 1 wherein the opposed end faces are each disposed at anonorthogonal angle α relative to a line perpendicular to a longitudinalaxis defined by said microresonator cavity.
 8. A microlaser according toclaim 7 wherein the opposed end faces are each disposed at the samenonorthogonal angle α such that the opposed end faces are parallel.
 9. Amicrolaser according to claim 7 wherein the opposed end faces areoriented in opposite directions by the same nonorthogonal angle α.
 10. Amicrolaser according to claim 7 wherein the opposed end faces are eachdisposed at an angle α that is between about 30° and about 45°.
 11. Amicrolaser according to claim 1 further comprising: a heat sink uponwhich at least said microresonator is mounted; and housing in which atleast said microresonator is disposed, said housing comprising a firstwindow through which laser signals generated by said microresonator areemitted.
 12. A microlaser according to claim 1 wherein saidmicroresonator is monolithic.
 13. A microlaser according to claim 1wherein said active gain medium is comprised of neodymium-doped yttriumaluminum garnet (YAG), and wherein said passive Q-switch is comprised oftetravalent chrome doped YAG.
 14. A microlaser according to claim 1wherein said active gain medium is comprised of thulium-doped yttriumvanadate (YVO₄), and wherein said passive Q-switch is comprised ofholmium-doped YVO₄.
 15. A microlaser comprising: a microresonatorcomprising an active gain medium and a passive Q-switch proximate saidactive gain medium, said microresonator extending lengthwise betweenopposed end faces; and first and second reflective surfaces disposedproximate respective ones of the opposed end faces to define amicroresonator cavity therebetween, said microresonator cavity defininga longitudinal axis extending between the opposed end faces, wherein theopposed end faces are each disposed at a nonorthogonal angle α relativeto a line perpendicular to the longitudinal axis defined by saidmicroresonator cavity such that the microlaser is capable of supportinga zig-zag resonation pattern in response to pumping of the active gainmedium via at least one of the end faces of said microresonator.
 16. Amicrolaser according to claim 15 wherein the passive Q-switch isadjacent one of the opposed end faces of the microresonator.
 17. Amicrolaser according to claim 15 wherein the opposed end faces are eachdisposed at the same nonorthogonal angle α such that the opposed endfaces are parallel.
 18. A microlaser according to claim 15 wherein theopposed end faces are oriented in opposite directions by the samenonorthogonal angle α.
 19. A microlaser according to claim 15 whereinthe opposed end faces are each disposed at an angle α that is betweenabout 30° and about 45°.
 20. A microlaser according to claim 15 furthercomprising an antireflection coating upon the end face through which thepump signals are delivered for permitting pump signals within apredetermined range of wavelengths to be received by the active gainmedium without being reflected from the end face.
 21. A microlaseraccording to claim 15 wherein said microresonator further comprises aplurality of side surfaces extending between the opposed end faces, andwherein the plurality of side surfaces are roughened to thereby diffuselight.
 22. A microlaser according to claim 15 wherein the microresonatoris adapted to generate laser signals of a predetermined wavelength, andwherein said first reflective surface is highly reflective for lasersignals having the predetermined wavelength while said second reflectivesurface is partially reflective for laser signals having thepredetermined wavelength, thereby permitting laser signals to be emittedby the microlaser via said second reflective surface.
 23. A microlaseraccording to claim 15 wherein said first and second reflective surfacesare coated upon respective ones of the opposed end faces of saidmicroresonator.
 24. A microlaser according to claim 15 wherein saidfirst and second reflective surfaces are spaced from respective ones ofthe opposed end faces of said microresonator.
 25. A microlaser accordingto claim 15 wherein said active gain medium is immediately adjacent saidpassive Q-switch.
 26. A microlaser according to claim 15 wherein saidactive gain medium is integral with said passive Q-switch.
 27. Amicrolaser according to claim 26 wherein said microresonator ismonolithic.
 28. A microlaser according to claim 15 wherein said activegain medium is comprised of neodymium-doped yttrium aluminum garnet(YAG), and wherein said passive Q-switch is comprised of tetravalentchrome doped YAG.
 29. A microlaser according to claim 15 wherein saidactive gain medium is comprised of thulium-doped yttrium vanadate(YVO₄), and wherein said passive Q-switch is comprised of holmium-dopedYVO₄.